Nitrite salts as poisons in baits for omnivores

ABSTRACT

The present invention relates to baits, especially baits to be used in humane methods for controlling feral omnivore populations. The invention also relates to methods of manufacturing the baits.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/449,806, filed Mar. 1, 2010, which is a U.S. National Stage patentapplication based on International Application No. PCT/AU2008/000260,filed Feb. 28, 2008, which claims priority to Australian Application No.2007901050, filed Feb. 28, 2007 and U.S. Application No. 60/903,891,filed Feb. 28, 2007, all of which are incorporated herein by referencein their entirety.

FIELD OF THE INVENTION

The present invention relates to baits, especially baits to be used inhumane methods for controlling feral omnivore populations. The inventionalso relates to methods of manufacturing the baits.

BACKGROUND OF THE INVENTION

Globally feral omnivorous animals cause remarkable environmental,biodiversity, agricultural and or industry damage. For example,omnivorous rodents are a global problem requiring vast resources tocontrol. In New Zealand, introduced brush tail possums now number anestimated 60-70 million. The damage they cause is both environmental andfinancial: they prey on eggs and chicks of native birds and on nativeinsects, damage native forests and spread bovine tuberculosis, posing animmense threat to dairy and beef industries.

In Australia, feral pigs are estimated to number in excess of 4 millionwith some estimates as high as 25 million. Feral pigs occupy some 40% ofthe land mass of Australia. These population estimates mean that theremay be more feral pigs in Australia than grazed cattle. Feral pigsinhabit, and are well adapted to, a wide range of habitats that includesub-alpine, temperate, sub-tropical, tropical and arid zones, and theyare present in most Australian states and territories. While brush-tailpossum over browsing of native and agriculturally important flora inTasmania is also of some concern.

Furthermore, in the United States, the presence of some 4 million feralpigs (also referred to as hogs, boar, or swine) has been reported insome 28 states ranging from California to Virginia, the majorityresiding in Texas. Feral pigs are the most abundant introduced ungulatein the United States.

Feral pigs have a very high fecundity and frequently raise litters inexcess of 6 piglets per breeding sow. Thus, the capacity for feral pigpopulations to respond to control measures or to totally exploit a foodsupply is large.

Feral pigs adversely impact agricultural production, environments andecosystems. A number of studies have identified a range of environmentaland agricultural problems arising from feral pig infestations (Alexiou(1984) Effects of feral pigs (Sus scrofa) on sub-alpine vegetation atSmokers Gap, ACT, Proceedings of Ecological Society of Australia, 12:135-142; Tisdell, C. A., (1982) Wild Pigs: Environmental Pest orEconomic Resource? Pergamon Press, Sydney; Miller, B. and Mullette, K.,(1985) Rehabilitation of an endangered Australian bird: the Lord HoweIsland Woodhen, Tricholimnas sybvestris, Biological Conservation, 34:55-95; Mitchell, J. and Mayer, R., (1997) Digging by feral pigs in thewet tropics world heritage area of north Queensland, Wildlife Research,24: 591-601; Choquenot, D., McIlroy, J. and Korn, T., (1996) ManagingVertebrate Pests: Feral Pigs (Ed. M. Bomford) Bureau of ResourceSciences, Australian Government Publishing Service, Can berra 163 pp;Mitchell, J., (2000) Ecology and management of feral pigs in tropicalrainforest, Unpublished PhD Thesis, James Cook University of NorthQueensland, Townsville; Hone, J., (2002) Feral pigs in Namadgi NationalPark: dynamics, impacts and management, Biological Conservation 105:231-242); Singer, F. J., Swank, W. T., and Clebsch, E. E. C., Effects ofwild pig rooting in a deciduous forest., Wildlife Management 48:464-473; Lacki, M. J., and Lancin, R. A., (1986) Effects of wild pigs onbeech growth in Great Smoky Mountains National Park, Journal of WildlifeManagement 50: 655-659. The key points from these various studies aresummarised below.

The predatory behaviour of feral pigs causes major economic damage foranimal production enterprises over wide areas. The damage is so severethat some areas cannot sustain productive grazing due solely to thewidespread infestation of feral pigs. The species' impact onagricultural production has been conservatively estimated to be inexcess of 100 million Australian dollars annually.

Feral pigs also cause significant damage to the environment due to theirdestructive foraging habits which include digging (rooting) andconsumption of plants. This destructive behaviour can damageinfrastructure including fencing, dams and levy banks and also causesdamage to wide areas of fragile riparian habitat. Their effect on nativeanimal species is unknown but is likely to be severe in view of theirpredatory behaviour and competition for food resources.

Apart from direct damage to grazing enterprises and the environment, thebiggest risk from feral pigs arise from their capacity to harbourseveral major human and animal diseases. Many diseases are zoonotic andthe pig provides an ideal amplifying host. Japanese encephalitis virus,leptospirosis, brucellosis and melioidosis have already been detected inferal pigs in Australia. However an even greater risk comes if there isan incursion of foot and mouth disease (FMD) virus into the feral pigpopulation, where the cloven hoofed pigs provide an amplifying host andvirus carrier that is widely distributed and highly mobile.

In the United States, pseudorabies virus (PRV) has been eradicated fromdomestic pigs however PRV continues to circulate in feral pig andraccoon populations. Accordingly, feral omnivore populations can also bea reservoir for fresh PRV outbreaks.

Details of the environmental, human health, animal health andagricultural production problems that arise already, or which mightarise, from an unchecked expansion in feral pig numbers are provided inthe book “Managing Vertebrate Pests: Feral Pigs,” (Choquenot, D.,McIlroy, J. and Korn, T., (1996) Managing Vertebrate Pests: Feral Pigs(Ed. M. Bomford) Bureau of Resource Sciences, Australian GovernmentPublishing Service, Can berra 163 pp. Infestation of other omnivorousspecies such as raccoons, collared peccaries, opossums, possums androdents can give rise to similar adverse agricultural, environmental,financial and health concerns in various countries.

There is therefore a considerable effort focussed on a reduction of therisks posed by feral omnivorous species in Australia, United States, NewZealand and other parts of the world which have unchecked populations ofsuch species.

Despite their impact, the control of omnivores such as feral pigs,possums, raccoons, and rodents is generally time-consuming, ad-hoc andreactive rather than pro-active management. Many techniques arecurrently employed for mainly localized control (eg shooting, trapping,fencing, etc) however it has been recognized that broad-scale andintegrated baiting campaigns are most cost-effective for reducing andmaintaining feral omnivore populations at low levels across large areas.Typical baiting campaigns include ground baiting and aerial baitingwhere the bait is dropped from an aircraft into the loci of the targetomnivore population to be controlled.

Lethal baiting campaigns include the use of various poisons, forinstance, sodium fluoroacetate (1080) which is placed in or deposited oncereal grains, fermented grain, compressed bran/pollard pellet baits,fresh or dried meat, offal, carcasses, lupin seeds, and fruit andvegetables, and also cyanide in manufactured baits. Of these, the use ofsoaked or dry wheat grain or fresh meat baits are the most common.

Of the various control means discussed above poison baiting of feral pigand other omnivore populations is recognised as one of the mosteffective means of controlling such populations and reducing the damagethey cause. Unfortunately however, one of the main problems with many ofthe bait types made from grains or meat and carcass offal or pellets isthat they exhibit poor target specificity. Accordingly, while thecommonly employed baiting campaigns may prove effective in controllingferal omnivore (eg pig) numbers in a particular area, such campaigns mayalso adversely affect individuals of other species of animals which maybe desired or native species of animals or birds which come into contactwith the baits.

Other disadvantages of the present baiting regime can be attributeddirectly to the specific poison used. For instance, a disadvantage of1080 is that feral pigs appear to be relatively resistant to the effectsof the poison compared to rabbits, foxes, and wild dogs for which it isa more ideal poison. For example, during captive trials with baitdelivered 1080, (Mcllroy et al, Australian Wildlife Research 16:195-202) dingos required 0.11 mg/kg to receive an LD₅₀ dose whereasferal pigs were reported to require at least 1 mg/kg and some as high asup to 4.11 mg/kg (O'Brien et al, Australian Wildlife Research 15:285-291).

Moreover, while the terminal toxic events associated with 1080 toxicosisare not thought to be accompanied by conscious pain, there aredisturbances in the central nervous system and thus effects on behaviourthat can appear unpleasant to the untrained observer. Humans that haverecovered from nearly lethal exposures to 1080 have not recalled painafter the event however the final phases of toxicosis have been likenedto hypoglycaemic or epileptic fitting. Sodium fluoracetate is presentlyone of the best toxin choices for feral pig management. However the highdoses required for feral pigs mean that this is not a perfectly suitabletoxin for pig management.

People poisoned with other poisons such as phosphorus (CSSP) orstrychnine have reported substantial pain and suffering and it is highlylikely that such poisons are too inhumane to be used to control feralanimals such as the pig. Similarly, while warfarin is usedtherapeutically in low doses for humans suffering from blood clottingdisorders and this use is not associated with pain, the use of thisanticoagulant in large animals such as feral pigs may give rise topainful haemorrhaging in some animals and therefore it is also not apreferred poison for this application.

This would indicate that none of these poisons and conventionally usedfood based baits, are perfectly suitable or represent perfectly humanealternatives for eradicating or controlling pest species.

The present invention serves to address at least some of theseshortcomings.

SUMMARY OF THE INVENTION

The present invention provides a bait for omnivorous feral animalscomprising a solid or semi-solid carrier material and a nitrite salt.

The invention also provides a method for humanely controlling omnivorousferal animals including the step of dispersing within the area offoraging of said omnivorous feral animals at least one bait comprising asolid or semi-solid carrier material and a nitrite salt.

The present invention further provides a method of preparing a bait foromnivorous feral animals comprising a solid or semi-solid carriermaterial and a nitrite salt, said method comprising:

-   -   (a) mixing carrier material components with a nitrite salt to        form a paste or suspended mixture;    -   (b) extruding the mixture into a casing; and    -   (c) allowing the mixture to at least partially solidify in the        casing thereby forming the bait.

BRIEF DESCRIPTION OF FIGURES

FIG. 1. A diagrammatic illustration of a bait of the present invention.

FIG. 2. A diagrammatic illustration of a bait of the present invention.

FIG. 3. Trace of blood Met Hb levels of feral pigs poisoned with sodiumnitrite. Plot of the % Met Hb level in blood as a function of time(minutes) for feral pigs poisoned with sodium nitrite at levels of 90,135, 180, and 180 (plus Na₂CO₃) respectively.

FIG. 4 a. Multidimensional scaling analysis trace highlighting thedissimilarity between feral pig dose rate groups base on all pair wisecoefficients as calculated with SIMPER analysis.

FIG. 4 b. Multidimensional scaling analysis trace highlighting thedissimilarity between feral pig dose rate groups base on all pair wisecoefficients as calculated with SIMPER analysis.

FIG. 5. Flow chart demonstrating experimental design of proof of concepttrial.

FIG. 6. Flow chart demonstrating experimental design of toxins in feedtrial.

DETAILED DESCRIPTION OF THE INVENTION

The term “bait” as used herein refers to the combination of apurposively selected carrier material and active agent for the expresspurpose of preparing a pest control agent wherein the carrier materialand the bait as a whole are palatable and at least partially edible by atarget pest. Accordingly, the “bait” of the present invention is apurposely manufactured pest control agent which is to be contrastedwith, for instance, a naturally occurring material (eg plant material)which may naturally contain quantities of the active agent.

The invention relates to the humane control of omnivorous feral animals,in particular, feral pigs. Accordingly, the terms “humane” and“humanely” as used herein refer to methods which do not cause unduedistress to the target animal species. Signs of distress which areavoided or minimised by the present invention include haemorrhaging,excessive vomiting, vocalisation, severe central nervous systemdisruption (including hyperexcitability, convulsions, ataxia, legtrembling and leg paddling whilst prone) and prolonged death. Preferablydeath occurs within 2-3 hours after ingestion of a bait of the presentinvention.

In relation to the present invention the target pests are omnivorousferal animals, preferably omnivorous feral mammals. More preferably theinvention is directed to the control of feral pig and possumpopulations.

It will be appreciated that the term “feral” as used herein refers tothe target pest species (ie omnivorous animals) which live wild suchthat their population or numbers cannot be easily controlled. InAustralia, for instance, many feral animals such as dogs, goats, cats,and pigs, were originally introduced during British settlement as eitherdomesticated species, species suitable for hunting, or where introducedfor the purpose of possibly controlling yet other pests. After escapinginto the wild such animals have become feral, adopting and flourishingto life unaided by human intervention. Many feral animals are introducedspecies and their presence in the wild is unwanted as they can adverselyaffect agricultural endeavours such as crop production and grazing.Feral animals which are introduced species are distinguished from nativeor domesticated species. These feral animals also invariably causeadverse environmental impact, especially as their populations increase.As such these feral animals have been classified as pests and it isdesired to keep populations of such animals to a minimum or, wherepossible, to completely eradicate them from the wild or from areas ofhigh agricultural or conservation valve. It will be understood thatwhile the bait of the present invention will not be able to distinguishbetween feral and non-feral domesticated animals, the bait is onlyintend to be used in the control of feral animal populations andaccordingly suitable measures should be taken to ensure that the bait isnot distributed amongst domesticated populations.

It will also be appreciated that “feral” also includes over-abundantnative species who's localised population may require control tomitigate the risk of disease spreading or over browsing of native and/oragriculturally important flora.

The term “active agent” referred to above is an agent which effects thephysiology of the target feral animal in a desired manner. The activeagent of the present invention is a nitrite salt. Nitrile salts includesodium nitrite, potassium nitrite, as well as amyl nitrite. Mostpreferably the active agent is sodium nitrite or potassium nitrite andmore preferably sodium nitrite. Nitrite salts to be used in the bait ofthe present invention are commercially available.

It will be appreciated that during typical baiting campaigns multiplebaits are dispersed within the area of foraging of the target animalspecies. A single bait may not provide a lethal dose to a single targetanimal even if completely consumed. Preferably however the nitrite saltis in an amount which provides a lethal dose to a target feral animal.That is, the quantity of nitrite salt in a single bait is such that itwill effectively kill a feral omnivorous animal.

It would be appreciated that the lethal dose of a nitrite salt to kill aferal omnivorous animal will typically depend on the species physiologyand weight. In respect of feral pigs the lethal dose is preferably atleast 135 mg/kg of nitrite salt. Accordingly, for a feral pig weighing60 kg, to provide a lethal dose a single bait would need to comprise atleast 8.1 g of the nitrile salt. More preferably the lethal dose isbetween 270-540 mg/kg. Accordingly, as most adult feral pigs encounteredunder typical conditions may weigh anywhere between approximately 30-300kg the preferred amount of nitrite salt in a single bait is between 8.1g-162 g. More preferably the preferred amount of nitrite salt in asingle bait is less than 30 g and even more preferably less than 20 g,for instance less than 15 g.

For feral possums the lethal dose is preferably also at least 135 mg/kgand more preferably at least 163 mg/kg. More preferably the lethal doseis between 400-600 mg/kg. The preferred amount of nitrite salt in asingle bait is between 1.9 g-4.9 g.

The nitrite salt may comprise up to 25% by weight of the total weight ofthe bait. Preferably the nitrite salt comprise up to 15% by weight ofthe total weight of the bait.

The bait of the present invention is also composed of a solid orsemi-solid carrier material. The carrier material is designed to beattractive, palatable and edible to the target feral animal and as suchwill generally contain or consist of a component, or mixture ofcomponents, which is a potential food source for the target feralanimal. In addition the bait should be of a consistency suitable forconsumption by the target animal but sufficiently robust as to enableeasy storage and transport and also preferably able to survive intactthe forces that arise on hitting the ground during aerial deploymentfrom fixed wing or helicopter aircraft.

For the present invention which is directed to the control of feralomnivores, such as feral pigs, the carrier material is selected fromfood sources which are attractive, palatable and edible to the targetomnivore. For example, the carrier material may contain or consist ofanimal or plant derived components and any combination of these.

Examples of suitable animal components include fish meal, bone, meat,offal, skin, egg, milk proteins, casein, and fat.

Examples of suitable plant derived components include pollard, bran,maize (corn), plant fibres, flour, fruit, vegetables, seeds, cereal andstraw.

Preferably the carrier material is selected with non-target species inmind such that the potential uptake of the bait by non-target species isreduced. Thus, the selection of the particular type and amount ofcomponents which make up the carrier material may vary depending on thenon-target species which are to be avoided. For instance, in the casewhere the non-target species are herbivores (e.g. specific bird species,marsupials, etc) the carrier material may be selected to comprise ofmainly animal derived components. This is particularly preferred whentargeting feral pigs in Australia where the majority of non-targetnative species have a restricted dietary range and where many non-targetspecies are obligate herbivores or graniferous birds that are lessattracted to am omnivore bait.

The carrier material may also include specific chemical attractants,such as flavourants or scented substances (odourants). The chemicalattractant may be a natural or artificial essence, such as banana,honey, aniseed, molasses, cinnamon oil and chocolate. The carriermaterial may also comprise other additives known in the art such ascolourants, preservatives, binders, fillers, pH adjusters and the like.For instance, in a preferred embodiment the carrier material contains acolourant (dye) which makes the bait green in colour to mask the baitfrom non-target species such as birds which generally have a preferencefor eating yellow and red coloured food consistent with ripe fruit.Also, preservatives and binding agents may be added to providemechanical strength to the finished bait and to reduce the risk ofpremature degradation on storage.

Furthermore, in order to increase the target specificity the bait mayalso include repellants of other non-target species such as methylanthranilate which is a known bird repellant.

Without wanting to be bound by theory it is believed that the nitritesalt in the bait acts by causing methaemoglobin (Met Hb) formation inred blood cells, which prevents oxygen transport, and at specific dosescauses rapid death by methaemoglobinaema. Accidental death by nitritepoisoning has been reported for domestic livestock such as pigs (see,for instance, Vyt, P et al, Viaams Diergeneeskundig Tijdschrift, 2005,74, 359-363; Gibson. R., The Veterinary Record, Mar. 22, 1975, p 270;McParland, P. J., et al, The Veterinary Record, Mar. 1, 1980, p 201;Counters, D. E., et al, The Veterinary Record, May 3, 1975, p 412;Winks, W. R., The Queensland Journal of Agricultural Science, Vol. 7,No. 1 and 2, March and June 1950, pp 1-14; and London, W. T., et al,J.A.V.M.A, Vol 150, No. 4, pp 398-402.

From a report on the study of methemoglobin formation and reduction invarious animals (see Smith & Butler., Am. J. Physiology. 210(2):347-350,1966) it appears that the susceptibility to Met Hb formation may berelated to the Met Hb reduction rate in such a way that a rapid Met Hbformation rate is offset by a rapid Met Hb reduction rate. In this studyit was observed that pigs were particularly susceptible tomethaemoglobinaema because of the pigs inability to effectively reduceMet Hb. The reason for this is that pigs possess uniquely low levels ofmethaemoglobin reductase which makes them highly susceptible tomethaemoglobin forming compounds.

Accordingly, the manufactured baits of the present invention areparticularly suitable for the control of omnivorous feral animals (likefor instance feral pigs and possums) that are sensitive tomethaemoglobinaemia due to physiologically low levels of methaemoglobinreductase or low activity of this enzyme.

The suspectibility of some omnivorous species to sodium nitrite (SN) hasbeen compiled by the present inventors and is outlined in Table 1 below:

TABLE 1 Approx- Approx- Approx- imate imate imate NADH- gavage baitlethal Methb lethal lethal Body SN Reductase dose dose size dose Specieslevels (mg/kg) (mg/kg) (kg) (gm) Marsupials Common 30.6 163 489 4 2.0brushtail possum Northern 52 232 696 2 1.4 brown bandicoot EutheriansPig 12 103 310 50 15.5 Rat 10 97 291 0.1 0.03 Raccoon — — 150 3 0.75Mouse 53 235 705 0.03 0.02

It has now also been identified that not only do nitrite salts act aseffective toxins, poisoning and death occur rapidly and relativelypainlessly. The mechanism of action provides the quick development ofanoxia in the brain due to the reduced oxygen carrying capacity ofmethaemoglobin induced by the nitrite. Thus one of the first symptoms ofthe toxicosis is the occurrence of unconsciousness, in much the same wayas carbon monoxide acts. Carbon monoxide has been used as a method tohumanely dispose of unwanted animals and is considered to be one of themost humane techniques available for this process. This is to becontrasted with the severe clinical symptoms experienced with warfarin(bleeding in various organs leads to pain, eg lameness, etc),phosphorous (eg liver failure leading to slow lingering death whichresults in feeling sick for a long period of time, etc) and 1080(unconsiousness, seizures and recovery in between seizures which oftencan lead to injury in between seizures) poisoning. Also the speed ofdeath is very quick with nitrite so any symptoms are only experiencedfor a short period of time. Accordingly, an advantage of the baits ofthe present invention are that they provide a more humane alternative toexisting feral omnivore baits.

Accordingly, in another aspect the present invention relates to a baitfor humanely controlling feral omnivores comprising a solid orsemi-solid carrier material and a nitrite salt wherein the nitrite saltis in an amount which provides a lethal dose to a feral omnivore.

In order to enhance or supplement the methaemoglobin formingcapabilities of the nitrite salt and hence the lethality of the baits ofthe present invention, the bait may also include additionalmethaemoglobin forming substances. Such substances include directoxidants of haemoglobin such as other nitrites like amyl nitrite, ormethylene blue, toluidine blue and sodium thiosulphate. Indirectoxidants may also be included such as aminophenols (for instance,o-aminophenyl, p-aminophenol, m-aminophenol), alanine, metaxylidine,hydroxylamines (for instance, phenylhydroxylamine), hydrazines,acetanilid, acetophenetidin, m-phenylenediamine, nitrosobenzene,nitrobenzenes (for instance, o-dinitrobenzene, p-dinitrobenzene,m-dinitrobenzene, trinitrobenzene, o-nitrotoluol, m-nitrotoluol,p-nitrotoluol, 2,4-dinitrotoluol, 2,6-dinitrotoluol,2,4,6-trinitrotoluol, m-chloronitrobenzene, m-aminonitrobenzene,2,4-dinitrochlorobenzene, and p-nitro-o-toluidine), sulphonamides (forinstance, sulphanilamide), quinones, p-aminoacetophenone, napthalene,dimethylaminopyridine (4-DMAP), aminopropiophenones (for instance,p-aminopropiophenone). These substances may individually or collectivelyenhance or potentiate the development of methaemoglobin or may slow theconversion (reduction) of induced methaemoglobin back into normalhaemoglobin.

The active agent may be used in combination with the carrier material inany manner which will allow for the delivery of the active agent to thetarget pest. For instance, the active agent may be incorporatesubstantially within the carrier material or coated on the surface ofthe carrier material.

In one embodiment the carrier material may be grain seeds which havebeen soaked or coated with the nitrite salt or intimately mixed with thenitrite salt.

In a further embodiment the bait of the present invention may take theform of a piece of meat (eg part of a carcass) in which the nitrite saltis coated to the surface of the meat or is incorporated within the meat,for instance, by injecting a solution or dispersion of the nitrite saltinto the meat.

In a preferred embodiment the nitrite salt is simply dispersedthroughout the carrier material to form the bait. More preferably thenitrite salt is evenly dispersed throughout the carrier material.Dispersion may be accomplished by simple mixing of the carrier materialwith the required amount of nitrite salt.

Bait 1 as shown in FIG. 1 is cylindrical in shape and includes a solidcarrier material 2 which has sodium nitrite 3 evenly dispersedthroughout. The solid carrier material, and hence the bait, isstrengthen by a thin casing 4 which covers the outer circumference of 1.

In another embodiment the solid or semi-solid carrier material forms theouter surface of the bait and may be in physical contact with a corewhich contains the nitrite salt. Thus in this embodiment the corecontaining the active agent is located within the solid carrier materialand not dispersed throughout. This may include a plurality of cores. Itwill be appreciated that the core containing the active ingredient ishoused internally of the carrier material such that preferably nosurface of the core protrudes from the carrier material. Preferably thecore is located centrally within the solid carrier material. Morepreferably the core is located centrally within the solid carriermaterial such that no surface of the core is exposed to the outsideenvironment. Therefore, the solid carrier material completely covers orsurrounds the core which is located therein.

Bait 5 as shown in FIG. 2 is cylindrical in shape and includes a solidcarrier material 6 in physical contact with and a centralised core 1which is completely covered or surrounded by the carrier material suchthat no surface of the core is exposed to the outside environment. Thesolid carrier material, and hence the bait, is strengthened by a thincasing 8 which covers the outer circumference of 5.

The main advantage of having the core located within the solid orsemi-solid carrier material is that smaller non-target animals oranimals that merely sample the bait carrier material cannot readilyaccess the core and hence the active agent. This makes the bait moretarget-specific.

It would also be appreciated that the phrase “the core contains anactive agent” means that the core does not, to any great extent, allowthe release of the active agent (nitrite salt) into the surroundingcarrier material. This feature will assist in preventing consumption bynon-target animals. Accordingly, the active agent is present in the coreas an encapsulated or bound active agent. Thus the core may be composedof any suitable material which is able to contain the active agent.However it is desirable that the core material is of a consistency andflavour that is readily consumed by the target animal. For instance thecore may include encapsulating matrix forming materials which are usedin, for instance, the pharmaceutical industry. Typical encapsulatingmaterials include, gelatin, carbohydrates (including polysaccharides andmonosaccharides), fatty acids, waxes and tallow.

As discussed above in one embodiment the bait of the present inventioncomprises a core which is preferably “in physical contact with a solidor semi-solid carrier material”. This means that there is no physicalbarrier separating the core from the carrier material. This becomes onlypossible because the core is made of suitable ingredients which do notallow the active agent (to any great extent) to permeate into thesurrounding carrier material. Thus another benefit of the presentinvention is that the core does not need to be coated with a coatingmaterial (eg, shellac material or acrylate polymer) in order to containthe active agent within the core. This allows for ease in manufacturingwhich inturn provides certain manufacturing cost benefits. This alsoincrease the likelihood of consumption by the target animals, forexample feral pigs, since pigs mouth the bait material before eating andcan reject hard or unnatural components within a bait.

In this embodiment the core may be made up of hydrophobic ingredientssuch that there is limited exchange of water between the core andcarrier material. It is also desirable for the core to be stable uponstorage but that it becomes vulnerable to weathering and biologicaldegradation in the field so that the baits do not persist in theenvironment for extended periods. It is also desirable that the core issufficiently thermally stable so that the core ingredients do not meltor leach into the carrier material when the baits are deployed in hotareas where there is direct exposure of the bait to summer sunshine. Thecore is composed of ingredients such that the active agent (nitritesalt) is contained therein and so the core maintains limited stabilitywhen the bait is exposed to normal environmental conditions.

As the core is also intend to be attractive, palatable and edible to thetarget animal, the core may also comprise attractants, such asflavourants or odourants. In addition the core may also comprise otheradditives known in the art such as preservatives, binders, humectants,fillers, pH adjusters and the like. For instance, in order to preservethe activity of the active agent upon storage, transport and use,antioxidants may be added to the core during manufacture of the bait.Typical antioxidants include sodium benzoate, sodium metabisulphite andthe like.

The preferred baits of the present invention may be formed byconventional techniques used for forming pellets or tablets in thepharmaceutical or agrochemical industry. It would be evident to theskilled person that the actual shape of the bait is not a crucialparameter and that any obtainable shape is within the scope of thepresent invention. However, a particular shape may be advantageous, whenfor instance, the bait is being used in an aerial baiting campaign. Forexample, a specific shape may act to enhance the aerodynamics orphysical strength of the bait and allow for more precise dispersement.

Also, the size and shape of the bait may further enhance targetspecificity. For instance, larger baits for larger animals woulddecrease the likelihood that smaller non-target species could carry thebait or that enough of the bait could be consumed to provide a lethaldose.

The preferred baits of the present invention may be formed, forinstance, by direct compression or by simple extrusion processes usingan Archimedes screw extruder.

In the latter system the extrusion is accomplished by an Archimedesscrew that compresses the carrier mixture through a shape forming nozzleor into an outer casing. The extruder carrier may be cut or divided asit is extruded or may be allowed to partially or fully solidify beforethe carrier is cut to desired lengths to provide baits of differentsizes or weights. The cutting may be achieved by slicing means (eg fixedknives) to control the length of the resulting baits. The diameter andlongitudinal shape of the bait pellets can be controlled by the diameterand shape of the nozzle apertures and the outer skin used to contain thebait.

To aid in the manufacturing process according to the above procedurefurther additives or excipients may be added such as water or binderssuch as starch, gelatin or gum arabica. Lubricants may also aid in theaforementioned pellet production.

In a further embodiment the carrier material is formed by initiallyweighing and blending all dry ingredients in a mixer/extruder. Theliquid ingredients are then added and mixed in order to form anextrudable paste or suspension. The nitrite salt may be added to thepaste and suspension and mixing continued to disperse the salt.

The mixture is then extruded into a thin casing in order to provide thecarrier material with a particular form once the carrier material setssolid. Preferred casings include non and semi-permeable natural andsynthetic polymer films which are edible such as synthetic cellulosefilm casings. The use of a casing in this manner is especially preferredwhere the bait is to be used in aerial baiting campaigns as the casingprovides the bait with added strength (impact resistance) when it isdispersed from the air. The casing is also preferred because it improvesthe ease of handling and also aids in reducing non-target uptake and mayalso be manufactured with printed labels to provide additional labellingand safety information as appropriate or to further camouflage the baitto reduce uptake by non-target species. Casings which are biodegradable(such as synthetic cellulose film or natural collagen casings) areadvantageous because they allow baits which have not been consumed, orpartially consumed, to degrade naturally, although it is recognised thatother materials such as plastic skins could be used. This also reducesthe likelihood of the bait being consumed by non-target species. In thisembodiment it is typically also preferred that the casing is coated withan attractant so that the bait retains its attractiveness to the targetanimal. For instance, the casing may be coated with fish oil or otherattractants.

The preferred baits of the present invention where the nitrite salt isevenly dispersed throughout the carrier material may be made by theabove process. For the embodiment where the nitrite salt is contained ina core, the manufacture of the shaped solid carrier material iscompleted, without the addition of nitrite salt and the core. This maybe done by punching a hole within the shaped solid or semi-solid carriermaterial to form a dosing well ready for core insertion. The ingredientsof the core (including the nitrite salt) may be introduced into the wellas a liquid, suspension, or paste which solidifies partially orcompletely after insertion. Alternatively, the core may be shaped andallowed to solidify (either partially or completely) and then insertedinto the dosing well. Accordingly, in an embodiment the core may beprepared by introducing the core ingredients into a casing material toform a core shape amendable to insertion into the dosing well. In thisembodiment the core casing would also need to be prepared from amaterial which is edible. Accordingly such an embodiment would mean thatthe core and carrier material are physically separated by casingmaterial.

Preferably, a plug of the same carrier material is then inserted intothe dosing well or the well otherwise closed over after dosing thecarrier material with the core components so that the core is notexposed to the external environment.

Accordingly, another aspect of the invention provides a bait foromnivorous feral animals with an outer edible layer comprising a coreand a solid or semi-solid carrier material wherein the core contains anitrite salt as an active agent and the core is located within the solidor semi-solid carrier material.

The present invention further provides a method of preparing a bait foromnivorous feral animals comprising a core in physical contact with asolid or semi-solid carrier material wherein the core contains a nitritesalt as an active agent and the core is located within the solid orsemi-solid carrier material, said method comprising:

-   -   (a) mixing carrier material components to form a paste or        suspended mixture;    -   (b) extruding the mixture into a casing;    -   (c) allowing the mixture to at least partially solidify in the        casing thereby forming the solid or semi-solid carrier material;    -   (d) forming a dosing well within the solid or semi-solid carrier        material;    -   (e) mixing core ingredients and active agent to form a liquid or        suspension or preparing a solid form core;    -   (f) partially filling the dosing well with the liquid,        suspension or solid form core of step (e); and    -   (g) plugging the remaining volume of the dosing well with the        solid or semi-solid carrier material.

The invention also provides a method for humanely controlling omnivorousferal animals including the step of dispersing within the area offoraging of said omnivorous feral animals at least one bait according tothe present invention.

The baits of the present invention are particularly suited for targetspecific ground or aerial baiting regimes. The method preferablyemployed includes identifying a target feral animal population anddistributing a quantity of the bait within the loci or area where theferal animals forage.

The quantity of bait distributed will depend on the number of targetanimals in the population to be controlled.

As mentioned previously one of the advantages of the bait of the presentinvention is that it may be presented in a form which is more targetspecific than existing baits. In order to further increase targetspecificity, one can design a baiting campaign which is better aimed atthe desired feral animal.

For instance, feral pigs are required to access water on a daily basis.Accordingly, baits may be dispersed around known watering points indrier areas or at drier times of the year. Also, feral pigs have anocturnal or crepuscular lifestyle, therefore dispersing baits in thelate afternoon may aid in minimising contact with non-target species.Furthermore, feral pigs are also known to have very large, nearlycompletely overlapping home ranges. Thus, placing groups of bait wellapart may also aid in minimising non-target fatalities.

The reference in this specification to any prior publication (orinformation derived from it), or to any matter which is known, is not,and should not be taken as an acknowledgment or admission or any form ofsuggestion that that prior publication (or information derived from it)or known matter forms part of the common general knowledge in the fieldof endeavour to which this specification relates.

Throughout this specification and the claims which follow, unless thecontext requires otherwise, the word “comprise”, and variations such as“comprises” and “comprising”, will be understood to imply the inclusionof a stated integer or step or group of integers or steps but not theexclusion of any other integer or step or group of integers or steps.

Certain embodiments of the invention will now be described withreference to the following examples which are intended for the purposeof illustration only and are not intended to limit the scope of thegenerality hereinbefore described.

EXAMPLES Example 1 Preparation of a Bait with a Hydrophobic Toxic Corea) Formulation Components

Sodium Nitrite sufficient to give 25 grams per bait, Wax, Tallow,Propylene glycol, Water, Salt, Sugar, Gelatine, Citric acid, Potassiumsorbate, Sodium benzoate, Sodium metabisulphite, Brilliant blue dye,Fish meal, Maize, Flour, Pollard, Tuna oil, and Fibrous casings (tunaoil coated).

b) Formulation Process Stage 1 Mixing

The dry ingredients for the carrier material may be first weighed thenblended dry for one minute in a two directional ribbon mixer/extruder.The tuna oil and tallow may be added followed by the pre-heated (80° C.)liquid ingredients (containing all water-soluble additives except thepoison) and mixing continued for 3 minutes until a uniform consistencyand colour is achieved.

Stage 2 Extrusion and Cooling

The warm mixture may be extruded into 55 mm diameter synthetic cellulosecasings, which may be coated with a small quantity of tuna oil, intolengths and allowed to cool and solidify to a firm rubbery consistencyovernight.

Stage 3 Cutting

The solidified long casing extrusions may be cut to the required lengthsto form shaped carrier material of approximately 240 grams weight thatare approximately 90 mm long and 55 mm diameter.

Stage 4 Formation of Dosing Well in Shaped Carrier Material for CoreInsertion

The cut and shaped carrier material may be transferred to a holepunching ram that forms a slightly tapered cylindrical dosing well ofapproximately 15 mm diameter through the centre of the carrier materialfrom one cut end, to within 2 cm of the bottom of the shaped carriermaterial.

Stage 5 Preparation of the Core Containing Sodium Nitrite

A freshly prepared suspension of sodium nitrite in a hydrophobic mixtureof wax and prime tallow may be formulated at 80° C. in a melted stateand transferred to a precision metering pump fitted with athermostatically controlled jacketed tank where the mixture isconstantly agitated while maintained at 70-80° C.

Stage 6 Dosing and Closing

The sodium nitrite suspension may then be pumped via a verniercalibrated metering pump to the bottom of the dosing well, where it canbe allowed to solidify. A 10 mL volume of the suspension may deliver anominal dose of 25 grams of sodium nitrite per bait. The open end of thedosing well may then be closed over with carrier material by a secondram to secure the poison core within the carrier material and away fromthe bait ends.

Stage 7 Packing and Batch Numbering

The finished baits may then be packed into high strength 20 litrepolypropylene tubs fitted with rubber sealed tamper evident lids andlabelled with approved labels and date of manufacture, batch number andexpiry date.

Example 2 Preparation of a Bait with a Sodium Nitrite/Honey Toxic Corea) Formulation Components

Sodium Nitrite sufficient to give 25 grams per bait, Propylene glycol,Water, Salt, Sugar, Gelatine, Honey, Citric acid, Potassium sorbate,Sodium benzoate, Sodium metabisulphite, Brilliant blue dye, Fish meal,Maize, Flour, Pollard, Tuna oil, and Fibrous casings (tuna oil coated).

b) Formulation Process Stage 1 Mixing

The dry ingredients for the carrier material may be first weighed thenblended dry for one minute in a two directional ribbon mixer/extruder.The tuna oil and tallow may be added followed by the pre-heated (80° C.)liquid ingredients (containing all water-soluble additives except thepoison and honey) and mixing continued for 3 minutes until a uniformconsistency and colour is achieved.

Stage 2 Extrusion and Cooling

The warm mixture may be extruded into 55 mm diameter synthetic cellulosecasings, which may be coated with a small quantity of tuna oil, intolengths and allowed to cool and solidify to a firm rubbery consistencyovernight.

Stage 3 Cutting

The solidified long casing extrusions may be cut to the required lengthsto form shaped carrier material of approximately 240 grams weight thatare approximately 90 mm long and 55 mm diameter.

Stage 4 Formation of Dosing Well in Shaped Carrier Material for CoreInsertion

The cut and shaped carrier material may be transferred to a holepunching ram that forms a slightly tapered cylindrical dosing well ofapproximately 15 mm diameter through the centre of the carrier materialfrom one cut end, to within 2 cm of the bottom of the shaped carriermaterial.

Stage 5 Preparation and Dosing of the Core Containing Sodium Nitrite

A freshly prepared mixture of sodium nitrite and honey may be added tothe dosing well.

Example 3 Preparation of a Bait with Toxin Dispersed Through Carrier a)Formulation Components

Sodium Nitrite sufficient to give 25 grams per bait, Propylene glycol,Water, Salt, Sugar, Gelatine, Citric acid, Potassium sorbate, Sodiumbenzoate, Sodium metabisulphite, Brilliant blue dye, Fish meal, Maize,Flour, Pollard, Tuna oil, and Fibrous casings (tuna oil coated).

b) Formulation Process Stage 1 Mixing

The dry ingredients for the carrier material including the sodiumnitrite may be first weighed then blended dry for one minute in a twodirectional ribbon mixer/extruder. The tuna oil may be added followed bythe pre-heated (80° C.) liquid ingredients and mixing continued for 3minutes until a uniform consistency and colour is achieved.

Stage 2 Extrusion and Cooling

The warm mixture may be extruded into 55 mm diameter synthetic cellulosecasings, which may be coated with a small quantity of tuna oil, intolengths and allowed to cool and solidify to a firm rubbery consistencyovernight.

Stage 3 Cutting

The solidified long casing extrusions may be cut to the required lengthsto form shaped carrier material of approximately 240 grams weight thatare approximately 90 mm long and 55 mm diameter.

Stage 4 Packing and Batch Numbering

The finished baits may then be packed into high strength 20 litrepolypropylene tubs fitted with rubber sealed tamper evident lids andlabelled with approved labels and date of manufacture, batch number andexpiry date.

Example 4 Trial Data Pen Trials

The captive feral pig colony at Robert Wicks Pest Animal Research Centre(Queensland Department of Natural Resource, Mines and Water) wascomprised of feral pigs harvested by commercial harvesters in the areasurrounding Inglewood, Queensland. A variety of sexes, ages and sizeswere drawn at random from the captive colony and used in these trials.Feral pigs were housed in the purpose built feral pig accommodation. Twodistinct trials occurred to test sodium nitrite as a poison for feralpigs. One trial delivered sodium nitrite to anaesthetised feral pigs bygavage (proof of concept trial), and a second trial delivered sodiumnitrite to feral pigs in food (bait delivery trial).

Proof of Concept Trial

This trial occurred in January 2006. The trial design is set out in FIG.5. Twenty-one fasted captive feral pigs were anaesthetised with ketamine(20 mgkg⁻¹) and xylazine (1.2 mgkg⁻¹) by intramuscular injection with a16 gauge needle into the quadriceps muscle. A known quantity of sodiumnitrite was prepared for each pre-weighed feral pig and dissolved orsuspended in isotonic water (15-70 mls). The resulting liquid was thenadministered by oesophageal gavage to an individual feral pig. A numberof sodium nitrate dosages were trialled on different groups of feralpigs. After administration, feral pigs were observed carefully forclinical signs and death. The dosing regime for sodium nitrite isoutlined below and in FIG. 5. Each dose rate contained three feral pigs.

Three untreated feral pigs functioned as placebos. They wereanaesthetised, gavaged with 15 ml of isotonic water, and underwentveni-puncture from the internal or external jugular vein. Specifically,an 18 gauge, 1 inch needle was inserted into the caudal part of thejugular groove near the manubrium and suction was provided by a 3 mlsyringe. The feral pigs were then allowed to recover from theanaesthetic in the same situation as other feral pigs in the trial, butwithout further veni-puncture. Control and recovered feral pigs wereeuthanased by a rifle shot. A post mortem examination was conducted onthese feral pigs and appropriate specimens collected.

Sodium nitrite (NaNO₂), referred to as SN hereafter, was administered totwelve anaesthetised feral pigs with or without sodium carbonate, bygavage with a stomach tube. This was done on an identical manner to theplacebo feral pigs. Specifically, three feral pigs received 30 g ofsodium carbonate (administered with 25 ml of isotonic water) followedimmediately by 180 mgkg⁻¹ of SN, dissolved in 15 ml of water. Threeferal pigs received only 18 mgkg⁻¹ of SN in 15 ml of water. Three feralpigs were administered SN at 135 mgkg⁻¹ in 15 ml of water. A final threeferal pigs were administered 90 mgkg⁻¹ of SN in 15 ml of water.

SN dosed feral pigs (not the 135 mgkg⁻¹ group) were blood sampled within1 min of administration of the toxin. Feral pigs were then sampledopportunistically at various times over the following minutes or hours.Smaller feral pigs were blood sampled more frequently, but were sedatedor were semi-conscious due to effects of SN when blood samplingoccurred. Feral pigs which recovered from anaesthetic quickly, or werelarge, were sampled less frequently than other feral pigs. All feralpigs were also blood sampled immediately after death (within 1 minute).The blood samples were used to assess Methaemoglobin levels (Met 315Hb). An automated radio-oximeter (model ABL520 supplied by RadiometerCopenhagen) was used for blood Met Hb analysis. The Met Hb data fromeach individual within a group was pooled to provide dose responsecurves for multiple feral pigs administered at a single dose rate. Theferal pigs which received 135 mgkg⁻¹ were not blood sampled until afterdeath. A post mortem examination was conducted on all feral pigs.

For SN poisoned feral pigs mean times to death and peak Met Hb levelswere calculated. A Kruskal-Wallis statistical test was used to determineany significant differences between the time to death at different dosesof SN, with or without sodium carbonate. Serial Met Hb levels werepooled for each group of feral pigs receiving a single dose rate andplotted against time. Trend-lines were fitted to the data. Logarithmicfunctions were used for the three higher dose rates, while a polynomialtrend line was fitted to the 90 mgkg⁻¹ feral pig group. This provided avisual representation of the effect of different doses of SN and thepossible synergist (sodium carbonate) on Met Hb levels. Multidimensionalscaling and ANOSIM tests were conducted on data generated from SN killedferal pigs using Primer (V.5) (Clarke and Gorley 2001 PRIMER V.5: Usermanual/tutorial. PRIMER-E: Plymouth UK.; Clarke and Warwick 2001 Changein marine communities: an approach to statistical analysis andinterpretation, 2nd edition, PRIMER-E Ltd, Plymouth UK). This programmakes few assumptions about data and consists of a range of univariate,multivariate and graphical routines for analysing matrices of samples.Specifically, ordination by multidimensional scaling (MDS) was used tovisually differentiate feral pigs based on variables (time to death,peak Met haemoglobin levels, time to dyspnoea and time to vomiting). Onefactor was used, dose rate. Data was normalized and 30 random resetsoccurred. ANOSIM was used to determine whether significant differencesbetween these variables were due to dose rate. SIMPER (a function ofPrimer) was used to determine dissimilarity between dose rates, or whatdose rates were most dissimilar.

Bait Delivery Trials

The trials were conducted in October 2005 and June 2006. Both trialswere designed to test the hypothesis that SN would be lethal to feralpigs when they consumed the toxin freely in food. During the secondtrial (see FIG. 6) to test sodium nitrite, 24 feral pigs were used.Before the toxin trial, each feral pig was pre-fed daily for 10 dayswith the bait substrate they were to receive during the toxin trial(bait or dry wheat). During the trial, six feral pigs acted as controls,receiving no toxin, but receiving bait substrate (carrier material).Three of these received wheat and three received the manufactured baitsof the type described in Example 2. The remaining 18 feral pigs weredivided into three groups of six animals. Three pigs in each groupreceived 250 or 500 g (depending on body size and therefore total toxicdose to deliver) of wheat and the other three received one or two thebaits (again, amount of toxin dependent). Each successive group of 6feral pigs received increasing doses of sodium nitrite at 135 (lowestsuccessful gavage dose), 270 and 540 mgkg⁻¹. Feral pigs were observedevery 15 minutes for 6 hours, then hourly for 6 hours, then ateuthanasia 12 hours after clinical signs developed (if they remainedalive). Each dose rate contained six animals divided evenly amongst twobait carrier materials. Six controls were used.

Results Pen Trials Proof of Concept Trial

Placebo Feral Pigs.

The placebo feral pigs all survived anaesthesia, blood sampling andgavage. The clinical signs were exclusively due to the anaesthesia.These included unconsciousness that lasted 30-50 min. All three animalsattempted to stand by 60 min, post administration of the anesthetic forsampling and all animals were standing normally by 75-120 min. At threehours after the injection of the anaesthetic agent, all feral pigs werestanding, walking, eating and drinking, and appeared normal. Theseanimals were euthanased by rifle shot at 3 hr post anaesthesia. Postmortem findings were unremarkable after a thorough examination ofthoracic and abdominal organs, and examination of skeletal muscles andsubcutaneous tissues. The only abnormality detected was small haematomasin the quadriceps associated with the injection of the anaestheticmaterial via jab pole.

Sodium Nitrite Feral Pigs.

Twelve feral pigs were administered SN (and three of these also receivedsodium carbonate). Initial Met Hb levels were close to zero for allferal pigs at the time of administration of SN. However, administrationof SN caused rapid increases in blood Met Hb values in all feral pigthat were sampled. The three higher doses appeared to cause more rapidand larger rises in blood Met Hb than the low dose (90 mgkg⁻¹, see FIG.3). The plotted line of best fit for each treatment group appears tosuit the data well with no r² value less than 0.88. Sodium carbonate didnot appear to act as a synergist with animals that received this havingsimilar curves to groups which received only high doses of SN.

All nine feral pigs that received doses of 135 mgkg⁻¹ or above died. Themean time to death was 106±75 (S.D.) min (range 42-130). The mean peakMet Hb level in killed feral pigs was 82%. Only one of three feral pigsadministered SN at 90 mgkg⁻¹ died. The single death occurred immediatelyfollowing blood sampling after Met Hb levels had begun to decline, andis possibly the result of oxidative stress during handling. Feral pigsthat were poisoned with higher doses appeared to die more quickly (seetable 2). However, times to death between the three lethal treatments(180 mg/kg, 180 mg/kg (with sodium carbonate) and 135 mg/kg) were notsignificantly different when examined with a Kruskal-Wallis test(K=5.067, d.f.=2, P=0.079).

TABLE 2 Dose rates, outcome and peak Met Hb levels¹ for each feral piggroup administered SN (see FIG. 3) Mean Weight time Mean Dose Sex ± topeak rate ratio S.D. Death/ death Met (mgkg⁻¹) (M:F) (kg) survival(minutes) hb (%)¹ 0 (placebo) 0:3 46 ± 12 0/3 NA NA 90/NaNO₂ 2:1 34 ± 301/2  NA² 55³ 135/NaNO₂ 1:2 35 ± 9  3/0 115 ± 18  89 ± 6 180/NaNO₂ 2:1 26± 16 3/0 87 ± 16 83 ± 8 180/NaNO₂ 2:1 30 ± 16 3/0 52 ± 26  80 ± 15 (withNa₂CO₃) ¹This is the approximate peak level since sampling time may nothave coincided with the time of peak Met Hb. However, after graphing andfitting a ‘line of best fit’ these values appear to be approximatelypeak values. ²The single individual that died took 302 minutes, althoughthe death was likely precipitated by handling during partialmetheamoglobinanemia. ³Two from three animals were assessed.

In the proof of concept trial it was sometimes difficult to distinguishwhich clinical signs were due to anaesthesia and which were due tointoxication. Comparison with placebo feral pigs assisted with this.There were two groups of feral pigs when considering clinical signs innitrite intoxicated feral pigs. One group exhibiting common signs werethe nine feral pigs which received ≧135 mgkg⁻¹, and the second groupwere the three feral pigs that received 90 mgkg⁻¹.

The feral pigs which received higher doses generally did not recoverfully from anaesthesia and died rapidly, with minimal clinical signs.Two of these nine animals recovered consciousness, and attempted tostand before the full effects of the SN became apparent and they againappeared to lose consciousness. Seven animals vomited between 1-4 timeswhen they were conscious, but retching was not prolonged. Most animalsbecame dyspnoeic approximately 30-60 minutes after gavage, and thisgradually worsened until marked gasping occurred just before death.In-coordination, paddling and short convulsive seizures occurred in twoof nine animals, and this was apparent close to the time when theseferal pigs died. The feral pigs which received lower doses showed aprolonged period of lethargy which lessened gradually over 4-6 hours.Vomiting occurred in two of three animals, three hours afterintoxication. One animal died approximately 5 hours after dosing,following blood sampling. Two feral pigs survived after receiving 90mg/kg doses of SN and had near zero Met Hb levels 14 hours postadministration.

MDS analysis confirmed that some dose rates produced syndromes that weredistinctly different to other feral pigs based on relative differencesin clinical signs, time to death and Met Hb levels (see FIG. 4 a).Extensive differentiation occurred with four individual feral pigs thatreceived high or low doses. However, the majority of feral pigs showedlittle differentiation with one another, and these individuals allreceived higher doses. Generally, higher doses tended to coincide withmore rapid times to death (see FIG. 4 b). Global ANOSIM analysis showedthat the difference between the groups of feral pigs receiving each dosewas significant (R=0.296, P=0.02). It was clear that biggestdissimilarity between the feral pigs was between the group that received90 mgkg⁻¹ and all other feral pigs when analysed with SIMPER. In otherwords, the average dissimilarity of all pair-wise coefficients betweenthe 90 mgkg⁻¹ feral pig and all other dose rates was approximately 23%(see table 3). The feral pig that received 90 mgkg⁻¹ and died tooklonger to vomit, had a lower peak Met Hb value and took longer to diethan the pigs which died after receiving the higher doses of SN (twoferal pigs of this dose rate did not die and are not analysed in theMDS). Higher doses resulted in shorter time to clinical signs and death.

TABLE 3 The dissimilarity between feral pig dose rate groups base on allpairwise coefficients as calculated with SIMPER analysis. The tabledemonstrates that the feral pig which received 90 mgkg⁻¹ was the mostdissimilar to other feral pigs which received higher doses. The higherthe dissimilarity figure, the more dissimilar are the pair-wisecomparisons. The group represented by 108S received 180 mgkg⁻¹ withsodium carbonate. Pair-wise dose rate comparisons (mgkg⁻¹) Dissimilarity180S vs. 180 10.44 180S vs. 135 7.82  180 vs. 135 13.08  90 vs. 13522.38   90 vs. 180S 20.55  90 vs. 180 25.76

Abnormalities were detected during post mortems in all animals whichdied following nitrite ingestion. The only consistent sign was dark orchocolate coloured blood and subsequent discolouration of organs andtissues which are well vascularised. Some animals (2) showed blood clotsor frank blood in various areas, generally the thorax. One feral pig hadprofuse bleeding into the thorax and this may have been sufficientlyextensive to contribute to death. This was a 44.8 kg male which died 47minutes after gavage with a peak Met Hb level of 90.1%. Most feral pigshad pale coloured lungs with some petechial haemorrhages. One animal hada partial thickness stomach ulcer with some associated bleeding on theomentum. This may have been an existing pathological feature or it mayhave been associated with passage of the gavage tube. In contrast noabnormalities were detected in the 90 mgkg⁻¹ group or placebo feralpigs.

Bait Delivery Trial.

Palatability.

At low doses (135 mgkg⁻¹) where toxins was concentrated with honey inthe middle of a bait (see Example 2), toxic baits were consumedreadility. At 270 and 540 mgkg⁻¹ dose rates, two from three animals ineach group consumed bait and toxin, with the third eating around thecentral honey/SN mass, and avoiding most of the toxin. At 135 mgkg⁻¹when toxin was mixed with honey, in a ‘clump’ in wheat, two from threeferal pigs refused any toxin, but ate the wheat around the toxic‘clump’. These two animals consumed the toxin readily when was dispersedthroughout bait substrate (carrier material). For the other feral pigsreceiving higher doses in wheat, the toxin was mixed throughout thewheat with a tablespoon of honey and all feral pigs consumed grain andtoxin readily.

Deaths.

Most animals completely consumed all bait material within 10-30 minutes.Six animals received 135 mgkg⁻¹ of SN in bait substrate, the lowesteffective gavage dose, but none died. Five animals consumed 270 mgkg⁻¹of SN and one of these died. The final animal from this 270 mg/kg dosegroup only consumed part of it's toxin in a bait and did not die. Allanimals consuming 540 mgkg⁻¹ in either bait substrate died (4/4; 2 foreach substrate). A fifth animal began showing clinical malaise whilst itwas still consuming wheat and only ate 370 g of the 500 g of presentedwheat but still died. Assuming even toxin dispersal throughout wheat itreceived approximately 400 mgkg⁻¹ of SN. The final animal from the highdose group consumed a partial dose (unknown amount) of SN in the baitand remained alive. The mean time to death for all dead animals was141±49 min (S.D.). The mean time to death for the animals that receivedthe full 540 mgkg⁻¹ dose was 122±49 min (S.D.). The mean time thatclinical signs were displayed by all animals was 84±41 min. The meantime that clinical signs were displayed by the animals receiving thefull 540 mgkg⁻¹ dose was 80±51 min. Animals that received sub-lethaldoses recovered after approximately 600±187 min (10±3 hrs). The meanweight of poisoned feral pigs was 25±11 kg with four males and twofemales.

The claims defining the invention are as follows:
 1. A bait foromnivorous feral animals comprising a solid or semi-solid carriermaterial and a nitrite salt.
 2. A bait according to claim 1 wherein thenitrite salt is sodium nitrite.
 3. A bait according to claim 1 whereinthe nitrite salt is in an amount which provides a lethal dose to a feralanimal.
 4. A bait according to claim 3 wherein the lethal dose is atleast 135 mg/kg of nitrite salt.
 5. A bait according to claim 1 whereinthe amount of nitrite salt is less than 30 g.
 6. A bait according toclaim 5 wherein the amount of nitrite salt is less than 15 g.
 7. A baitaccording to claim 1 wherein the omnivorous feral animals are feralpigs.
 8. A bait according to claim 1 wherein the omnivorous feralanimals are feral possums.
 9. A bait according to claim 1 wherein thenitrite salt is dispersed throughout the carrier material.
 10. A baitaccording to claim 9 wherein the carrier material comprises animalderived components.
 11. A bait according to claim 9 wherein the carriermaterial comprises fish meal.
 12. A bait according to claim 9 whereinthe bait is cylindrical in shape and is strengthened by a thin casingwhich covers the outer circumference of the bait.
 13. A bait foromnivorous feral animals with an outer edible layer comprising a coreand a solid or semi-solid carrier material wherein the core contains anitrite salt as an active agent and the core is located within the solidor semi-solid carrier material.
 14. A bait for humanely controllingomnivorous feral animals comprising a solid or semi-solid carriermaterial and a nitrite salt wherein the nitrite salt is in an amountwhich provides a lethal dose to an omnivorous feral animal.
 15. A baitaccording to claim 13 wherein the omnivorous feral animals areomnivorous feral pigs.
 16. A bait according to claim 13 wherein theomnivorous feral animals are omnivorous feral possums.
 17. A bait forhumanely controlling a population of omnivorous feral animals, said baitcomprising a solid or semi-solid carrier material and a nitrite salt,wherein the bait has an amount of nitrite salt which delivers at least135 mg of nitrite per kg based on the weight of a feral animal in saidpopulation, the nitrite salt comprising at least 3% by weight of thetotal weight of the bait, and wherein the nitrite salt is present in thebait in an encapsulated form.
 18. A bait according to claim 17 whereinthe omnivorous feral animals are omnivorous feral pigs.